Electrochemical double layer capacitors, including ultracapacitors and supercapacitors, are energy storage devices having an extremely large capacitance on the order of several Farads or more. Charge storage in double-layer electrochemical capacitors is a surface phenomenon that occurs at the interface between the electrodes, typically carbon, and the electrolyte. In double layer capacitors, the separator absorbs and retains the electrolyte thereby maintaining close contact between the electrolyte and the electrodes. The role of the separator is to electrically insulate the positive electrode from the negative electrode and to facilitate the transfer of ions in the electrolyte, during charging and discharging.
Electrochemical double layer capacitors are commonly used in applications which require a burst of power and quick charging; therefore it is desired to lower the ionic resistance within the capacitor and to increase the capacitance per unit volume. If the ionic resistance of the separator is too high, then during high current charging and discharging, the voltage drop will be significant resulting in poor power and energy output. It would be desirable to have a separator having reduced thickness with high porosity and low resistance, yet still able to maintain its insulating properties by keeping the positive and negative electrodes apart thus avoiding the development of short-circuits, which can ultimately lead to self-discharge. Capacitor separators should obstruct the electrophoretic migration of charged carbon particles released from one of the electrodes towards the other electrode, referred to as a “soft short-circuit” or “soft short,” to reduce the likelihood of self-discharge. Such obstruction is also referred to herein as “soft short barrier.” As electrochemical double layer capacitors are typically made in a cylindrically wound design in which the two carbon electrodes and separators are wound together, separators having high strength are desired to avoid short-circuits between the two electrodes. Additionally, as the capacitance of the capacitor depends on the amount of active material present within the volume of the capacitor, a thinner separator is desired.
Conventional double layer capacitor separators include wet-laid cellulose based paper that are not stable at high temperature (i.e., greater than 140° C.) or high voltage (i.e., greater than 3 V) and have unacceptable moisture adsorption. Impurities present in the separator cause problems at higher voltages. Microporous films have also been used, but have undesirably high ionic resistance and poor high temperature stability. It would be desirable to have capacitor separators with improved combinations of stability at high temperature and voltage, barrier to the electrophoretic migration of particles from one electrode to the other, lower ionic resistance and higher strength.